Electric motors are frequently used in the automotive field not only in electric cars but also as actuators in servocontrol systems for driving pumps, compressors and the like. Many functional devices of a car are motorized for enhanced performance, comfort and safety of the driver and of passengers, such as the fuel pump, the over-modulation compressor, the transmission gears, the power steering, safety belts, windows and so forth.
Some of these functional devices were servo-assisted by exploiting the motor torque available on the shaft of the thermal engine of the car. This implies inefficiencies in using part of the mechanical energy obtained by burning fuel for powering these devices. Lately the use of electric motors, that convert electric energy into mechanical energy, has become more and more the preferred choice. Permanent magnet brushless motors are often preferred in these automotive applications. The characteristics of permanent magnet brushless motors are particularly suitable for automotive applications: they are relatively light, have a high power density, a small size and are very reliable, though they are relatively expensive (due to the costs of the magnets used for fabricating the rotor), more difficult to control than a traditional DC electrical machine, and need more or less precise angular position sensors, depending on the brushless motor and the control strategy.
In particular, when implementing control strategies that contemplate a sinusoidal modulation of the PWM (Pulse Width Modulation), whether DC (trapezoidal induced back electromotive force) or AC (sinusoidal induced back electromotive force), it may be necessary to use sensors of the angular position, such as (incremental or absolute) encoders and resolvers that are relatively expensive (encoders) and use complicated sensing techniques (resolvers). In applications where it may not be necessary to gather information about the angular position of the rotor with a high precision (electro-hydraulic actuators, control systems with low dynamical performances requirements etc.) but where robustness, low cost and encumbrance are of paramount importance, the use of Hall sensors may be convenient.
A Hall sensor may be viewed as a thin foil of conducting material in which a current I flows in presence of an external magnetic field orthogonal to current lines in the foil, as depicted in FIG. 1. When the applied magnetic field is null, the current distribution over the foil is uniform and between the output terminals there is not any voltage. If, as shown in FIG. 2, a non null magnetic field is applied orthogonally to the surface, a certain voltage VH may be measured between the output terminals because of the effect of Lorentz force on the electrons.
This force alters the current distribution and the generated voltage VH is given by the following equation:
                              V          H                =                              1                          n              ·              q                                ⁢                                    I              ·              B                        b                                              (        1        )            wherein    I=applied current;    B magnetic induction;    q=electron charge;    n=carrier density; and    b=thickness of the foil along the direction of the magnetic induction B.
Commonly, as depicted in FIG. 3, brushless motors should have three or more Hall sensors. The sensors may be mounted at an extremity of the drive shaft, directly in the motor or around a magnetic ring mounted on the drive shaft. The system is realized such that, when a south pole of the permanent magnet rotor pass over the surface of the sensor, the latter generates a voltage pulse, that switches low when a north pole passes over the surface of the sensor. For simplicity sake, hereinafter reference will be made to the case in which only three Hall sensors are installed in the motor uniformly spaced of a same rotation angle of the rotor, but what will be stated holds similarly if more or less than three Hall sensors are installed.
FIG. 4 depicts sample waveforms of voltage signals generated by Hall sensors for a PMSM (Permanent Magnet Synchronous Motor) with two polar couples. Of course, the number of polar couples influences the number of switchings for each complete mechanical revolution of the rotor. For each couple of poles, there are six switchings of the Hall sensors, two for each phase; this implies that for a motor with two polar couples, there will be 6×2 switchings.
In correspondence of each leading or trailing edge of the pulse signals generated by the Hall sensors, it is possible to establish with precision that the rotor has rotated by 60 electrical degrees, but the signals generated by Hall sensors do not indicate precisely which is the position of the rotor between two successive switching edges. In literature, various ways of obtaining information on the angular position of the rotor of a brushless motor by using Hall sensors are described.
In the so-called step-mode control, the turn on and turn off instant of the components of an inverter are determined by the occurrence of the high pulse in the signal generated by the Hall sensors of FIG. 5. This method is simple to implement, but it has the following drawbacks: low resolution in the application of control vectors that cause surges of “cogging” torques; generation of noise at audible frequencies; and generation of EMI.
This control technique may be easily implemented with a hardware system. An example is given by the integrated circuit L6235, manufactured by STMicroelectronics, see DMOS driver for Three-Phase Brushless DC Moto and STMicroelectronics Datasheet L6235, September 2003 V. Marano, L6235 Three Phase Brushless Motor Driver, STMicroelectronics Application Note AN1625, October 2003. It includes a three-phase DMOS driver completely integrated in Multipower BCD technology and the logic circuit for controlling in step-mode the inverter integrated in the chip.
Moreover, STMicroelectronics has included in microcontrollers devices of the ST7MC family, 8-bit MCU with Nested Interrupts, Flash, 10-bit ADC, Brushless Motor Control, Five Timers, SPI, LINSCI, STMicroelectronics Datasheet ST7MC1/ST7MC2, November 2005, a dedicated peripheral for implementing a step-mode control, using Hall sensors. International Rectifier produces similar devices, called OMC506, Closed Loop Speed Controller for 3-Phase Brushless DC Motor MP-3T Packag, International Rectifier Datasheet OMC506, November 2003.
Another approach consists in estimating the electrical angular position and the mechanical angular speed by means of various reconstruction and interpolation algorithms starting from signals coming from Hall sensors. In Ting-Yu Chang et al., A Hall Sensor Based IPM Traction Motor Drive, IEEE 0-7803-7369-3/02/$17, 2002, a high frequency train of pulses phase-locked by a PLL is generated from Hall sensor signals. With this technique it is possible to increase the angular resolution up to 1.25 electrical degrees. No provision for managing the rotation speed in both senses nor any method for estimating the angular speed are proposed in the article.
The article J. X. Sheng, Z. Q. Zhu, D. Howe, PM Brushless Drives with Low-Cost and Low-Resolution Position Sensor, IEEE, discloses a technique for estimating the angular speed starting from signals coming from Hall sensors (every 60 electrical degrees). In order to take into account the fact that the sensors are never perfectly symmetrically installed on the motor, the calculation is not carried out on each single pulse of the signal coming from Hall sensors, but on a whole cycle and for each sensor such to have a mean value of such a speed jitter filtered information. The position is estimated by time integrating the speed using as initial condition the information about the position provided by the Hall sensors, each 60 electrical degrees.
The article ST7MC PMAC Sine Wave Control Software Library, STMicroelectronics, Application Note, AN1947/1204 Rev.1.0 discloses a technique that exploits the synchronism, typical of brushless motors, that consists in synchronizing the excitation with the position and the instantaneous frequency of the rotor. The most straightforward mode for obtaining this result consists in continuously measuring the absolute angular position of the rotor and its rotational speed, such that the excitation may be managed on the single phases of the motor with an exact synchronism with the motion of the rotor.
This is known as self-synchronization technique and uses as a feedback signal, a signal representative of the angular position of the rotor to prevent loss of synchronization. In this case, the Hall sensors are used for measuring the angular position of the rotor starting from the direction of the magnetic flux caused by the rotation of the permanent magnets.
The motor is powered with a sinusoidal voltage of amplitude A, frequency f and phase Φ. The motor so powered is spun at a frequency f. For each control cycle, in order to keep synchronism, the rotor speed is calculated, by considering the time elapsed between two successive pulses of the Hall sensors, and the position is updated. The position is updated every 60 electrical degrees and no technique is proposed for estimating the position of the motor between two edges of the Hall sensors. Yet a different approach from that of Ting-Yu Chang et al., A Hall Sensor Based IPM Traction Motor Drive, IEEE 0-7803-7369-3/02/$17, 2002, J. X. Sheng, Z. Q. Zhu, D. Howe, PM Brushless Drives with Low-Cost and Low-Resolution Position Sensor, IEEE and ST7MC PMAC Sine Wave Control Software Library, STMicroelectronics, Application Note, AN1947/1204 Rev.1.0 is disclosed in Jiri Riba, Sine Voltage Powered 3-Phase Permanent Magnet Motor with Hall sensor, Freescale Semiconductor inc., AN2357, 11/2002.